This Application relates to rotary printing machines, and more particularly, to printing machines which transport individual sheets of material to be imprinted from one stage of the machine to another by vacuum transfer systems, and in which dust contamination of the printing plates is a serious problem.
As disclosed, for example, in parent application Ser. No. 09/708,156, and in U.S. Pat. No. 5,564,693 incorporated by reference herein, it is known to transport individual sheets of material, such as corrugated cardboard, from one stage of printing machines to the next by means of transfer systems employing conveyor belts and/or rollers. In order to maintain the sheets in firm contact with the belts and/or rollers it is known to create a pressure differential above and below the plane of travel of the sheets which is known as the xe2x80x9cboard linexe2x80x9d. While maximum pressure differentials across the board line have been desired, maximum differential pressures have been limited by unacceptable levels of energy consumption in the motor-driven blowers. Also, maximum differential pressures have been limited by unacceptable levels of dust and other foreign particles, hereinafter xe2x80x9cdustxe2x80x9d, being drawn into the housing of the printing machine where they may foul the printing plates and create other serious operating problems. While dust is a significant problem in all printing machines, it is a major problem in printing machines which include die cutter sections in which the printed material is cut and scored such that large amounts of dust are created in the machine.
The present invention enables highly efficient pressure differentials to be created with low power levels, and with minimized dust flow adjacent the printing cylinders, by the controlled supply of supplemental air at negative pressure and at specific locations in the vacuum transfer system.